Polarizers are well known devices in classical optics, and are used to preferentially attenuate light/electromagnetic radiation polarized in one direction (along one of the principal polarization axes of the polarizer) and allow transmission of the orthogonally polarized radiation.
There are several types of polarizers available including:                i) prism polarizers, such as the Glan-type prism polarizers and the Nicol-type prism polarizers,        ii) polarizing beam splitters, such as the Rochon, Senarmont, and Wollaston polarizers,        iii) dichroic polarizers, which are based on the fact that dichroic material absorbs light polarized in one direction more strongly than light polarized at right angles to that direction. The most common materials used as dichroic polarizers are stretched polyvinyl alcohol sheets treated with absorbing dyes or polymeric iodine, commonly marketed under the trade name Polaroid, and        iv) wire-grid and grating polarizers, which transmit radiation whose E vector is vibrating perpendicular to the grid wires and reflect radiation with the E vector vibrating parallel to the wires when the wavelength λ is much longer than the grid spacing d. When λ is comparable to d, both polarization components are transmitted).        
A linear polarizer is any device which, when placed in an incident unpolarized beam, transmits a beam of light whose electric vector is vibrating primarily in one plane, with only a small component vibrating in the plane perpendicular thereto. If a polarizer is placed in a plane-polarized beam and is rotated about an axis parallel to the beam direction, the transmittance of the plane-polarized beam, T will vary between a maximum value T1 and a minimum value T2 according to the law:T=(T1−T2)cos2(θ)+T2.
where T1 and T2 are called the principal transmittances, in general T1>>T2; θ is the angle between the plane of the principal transmittance T1 and the plane of vibration of the electric vector of the incident beam.
The ratio of minimum transmission to maximum transmission of a polarizer as a function of the direction of linear polarization of the incident radiation beam is known as the extinction ratio of the polarizer given by the expressionρ=T2/T1.
It is often advantageous to make use of optical fiber/waveguide polarizers in optical systems, such as communication systems and sensor systems, whenever the systems incorporate optical fiber waveguides and planar waveguide devices in their architecture.
Polarizers are necessary to implement polarization-sensitive devices, such as many electro-optic modulators, and in polarization-sensitive applications, such as fiber gyroscopes. Forming overlays that selectively couple one polarization out of the guide can form polarizers on dielectric waveguides that support both transverse electric (TE) and transverse magnetic (TM) propagation. For example, a plasmon polarizer, formed on LiNbO3 by coating over the guide with a Si3N4/Au/Ag thin-film sandwich, selectively attenuates the TM mode. In some materials it is possible to form waveguides that only support one polarization (the other polarization is not guided and any light so polarized radiates into the substrate). In fact, one of the earliest fiber polarizers that was demonstrated made use of a highly birefringent fiber that supported only one polarization. By inserting short (mm) lengths of such guides in circuits or alternatively forming entire circuits from these polarizing guides, high extinction ratios can be obtained. For example, annealed proton exchange (APE) waveguides in LiNbO3 exhibit polarization extinction ratios of at least 60 dB. These devices are complex and expensive and suffer from an index of refraction mismatch with optical fiber waveguides.
Optical fiber/waveguide polarizers are available that share their operating principles with the polarizers of classical bulk optics. For example, polarizing dichroic material can be introduced between two axially aligned optical waveguides. The polarizing dichroic material will polarize light transmitted across the junction between the waveguides. However, such a device has some intrinsic limitations. The extinction ratio of the device will be limited by the optical properties of the polarizing dichroic material taken together with the maximum thickness of the material that can be introduced between the two optically connected waveguides. It is well known that bridging losses increase with separation between optically connected waveguides. Therefore, the maximum thickness of polarizing dichroic material that can be used is limited by the need to ensure that the bridging losses between the waveguides are kept below an acceptable value.
The limitation on the allowable gap between optically connected waveguides can be mitigated by using the beam expansion methods of micro-optics. By collimating the beam radiated by the transmitting waveguide using a lens, and refocusing the collimated beam with a second lens into the receiving waveguide, a tolerable gap is created between the lenses. The longer the focal length of the lenses, the greater is the gap size. In this type of configuration all that is needed to make a polarizer is to introduce between the collimating lenses any one of the classical bulk optic polarizers, such as prism polarizer or a polarizing beam splitter. The micro-optic approach suffers from the need to maintain critical alignment of all the optical components that comprise it. Furthermore, the devices are relatively bulky in practice.
Optical fiber that is tightly wound around a mandrel suffers from radiation losses. For a given principal polarization, these losses increase suddenly as the wavelength of the transmitted light increases above a characteristic wavelength value. The value of such a characteristic wavelength is polarization-dependent. Therefore, a properly wound fiber will act as a polarizer over a spectral range (between the two characteristic wavelength values associated with the two principal polarization states) where one polarization mode is lossy and the orthogonal polarization mode is relatively lossless. This device is bulky and suffers from performance limitations including narrow-bandwidth operation.
Another form of optical waveguide polarizer is based on a fused bi-conical taper coupler with polarization-dependent coupling characteristics. Ensuring that light of one polarization couples over while the light of orthogonal polarization does not, is all that is necessary to implement such a polarizer. In practice these polarizers suffer from poor extinction ratio and are usually narrowband in operation.
Fiber polarizers can also be made based on the polarizing properties of polymer-dispersed liquid crystals. Such a liquid crystal is placed between two coaxially aligned fibers. The liquid crystal scatters one polarization and transmits the other.
The cylindrical symmetry of an optical fiber leads to a natural decoupling of the radial and tangential components of the electric field vector. These polarizations are, however, so nearly degenerate that a fiber of circular symmetry is generally described in terms of orthogonal linear polarizations. This near-degeneracy is easily broken by any stresses or imperfections, which break the cylindrical symmetry of the fiber. Any such breaking of symmetry (which may arise accidentally or be introduced intentionally in the fabrication process) will result in two orthogonally polarized modes with slightly different propagation constants. These two modes need not be linearly polarized; in general, they are two elliptical polarizations. Such polarization splitting is referred to as birefringence.
U.S. Pat. No. 6,430,342, incorporated herein by reference, in the name of Kim issued Aug. 6, 2002, discloses a device having a mechanical fiber grating that can serve as an optical filter, such as a polarizer. In particular, the fiber grating according to Kim's disclosure has asymmetric mode coupling characteristics, so that it can be prevalently applied to an optical fiber notch filter, an optical fiber polarizer, an optical fiber wavelength tunable bandpass filter, an optical fiber frequency shifter and so on. Although Kim's device appears to perform its intended function, it is thought to be less than optimal as it is based on mechanically deforming the fiber to achieve these results.
More specifically, Kim's description of the best mode for carrying out the invention involves the mechanical formation of stepped microbends. Firstly a small portion of fiber section is melted with an electric arc discharge while it has been placed under shear bending stress. Upon cooling, the microbend deformation becomes permanent. The process is repeated at approximate beat-length intervals to create a long-period grating with many microbends. There are several disadvantages to devices fabricated using this structure and process. Firstly, because the grating consists of many microbends, it is necessarily long, so the stressing conditions can change appreciably as the process progresses along the fiber axis, due to the varying distance from the respective fixing boards 110 and 112 in FIG. 6B respectively. This compromises the precise control of the process, primarily the stress magnitude and the repeatability, thereby potentially reducing the reliability and manufacturing yield. Furthermore, using an electric discharge arc for locally heating the optical fiber has certain positional precision problems associated with it. It is also known that microbends cause scattering or out-coupling of both polarisation states from the optical fiber, which tends to increase the overall insertion loss in the device. As Gambling et al. describe in Optical and Quantum Electronics Vol. 11, pages 43-59, 1979, not only does the radius of curvature of the microbend affect the bending loss, but also the transition from a straight portion of fiber to a curved portion can give rise to additional loss, known as “transition loss” due to mode conversion and energy redistribution. Controlling the geometry of such transitions requires a very complex fabrication process, which may not be practical for commercial manufacturing. It is important to note that while both types of the above losses posses some polarization dependence which can be utilized for the fabrication, both add to the overall losses in the device. Finally, in the finished device the microbends created in the fabrication process constitute discontinuities in the fiber profile, probably creating mechanical stress concentration spots as well as points of residual stress, whereby the device reliability and robustness is impacted negatively.
In contrast, the instant disclosure teaches the use of a laser beam with attendant improved precision and control of the fabrication process. Furthermore, the described polariser retains its essentially straight geometry, thus reducing the likelihood of unwanted stress concentrations and practically eliminating the insertion losses consisting of bending loss and transition loss.
It is an object of this invention to provide a grating that is photo-induced resulting in an inexpensive, reliable high performance in-fiber polarizer.
Modes of an Optical Fiber
An optical fiber generally supports three types of modes: core-guided modes, cladding-guided modes and radiation modes. The modes of an optical fiber are the characteristic solutions of Maxwell's electromagnetic field equations for an optical fiber geometry that is invariant in translation along its longitudinal axis. For a given optical frequency, the modes of an optical fiber obey boundary conditions that ensure the continuity of the tangential component of the electrical and magnetic field vectors at all the boundaries. As well, the modal solutions must meet the requirement that all the modes that the fiber supports be restricted to carry a finite amount of power. The optical power carried by a core-guided mode is confined mainly to the core. The diameter of the core is usually about ten times less than the diameter of the cladding. The power carried by a cladding-guided mode is confined mainly to the cladding. The power carried by a radiation mode in the radiation continuum is not bound to the optical fiber. The physical quantity that determines the type of mode in question is the effective index of the mode. The effective index of the modes is an eigenvalue, which is obtained from the electromagnetic field equation solutions for the optical fiber structure. The effective, index of a core-guided mode lies between the refractive index of the core and the refractive index of the cladding in the case of a three-layer step index optical fiber consisting of a core, a cladding and a surrounding medium, that is, an outer cladding. The effective index of a cladding-guided mode for the same structure lies between the refractive index of the cladding and of the medium surrounding the fiber, that is, the outer cladding. An optical fiber is monomode if it supports only one core-guided mode. The condition for monomode propagation in the case of a three-layer step index optical fiber is well known: the normalized frequency of the core must be less than approximately 2.405. If the normalized frequency of the core becomes less than approximately 1.0 the light is, no longer guided primarily in the core and becomes cladding guided.
Without loss of generality, we can describe the invention with reference to commercially available monomode optical fibers. It will be clear to anyone familiar with the state-of-the-art that the general teachings of the invention will apply to other optical fibers as well, including multimode optical fibers and polarization-maintaining optical fibers.
Typical Optical Fiber
A typical optical fiber for use in optical communication systems is fabricated using low loss dielectric materials, usually high-purity fused silica and doped fused silica glass. The function of the dopant is to create the index of refraction contrast that differentiates optically the core region of the fiber from the cladding region. Usually the cladding is made of pure fused silica and the core is made from Germanium-doped silica. The effect of the Germanium dopant is to raise the index of refraction of the fused silica in the core. Thus, the fiber consists of a high refractive index core and a low refractive index cladding. Such optical fibers are commercially available. For example, Corning Inc. manufactures SMF-28 fiber (which is used extensively) that has low attenuation in the 1310 nm and the 1550 m transmission windows. The fiber supports a single (polarization independent) optical mode of propagation and is suitable for use in optical communication systems. Such a fiber has the following approximate characteristics:                Core radius: 4.15 microns        Cladding radius: 62.5 microns        Core index of refraction at (1310 nm): 1.4519        Cladding index of refraction (1310 nm): 1.4468        High degree of circular symmetry        Low transmission loss (less than 0.5 dB/Km)        
The polarizer, according to the present invention, can be made in such an optical fiber/waveguide.
Optical fibers with more complex index-of-refraction profiles than those of a three-layer refractive index profile fiber do exist; however the general method of fabricating an optical fiber according to the present invention applies also to such fibers with obvious modifications.
Birefringence
Crystalline materials may have different indices of refraction associated with different crystallographic directions. Commonly, mineral crystals having two distinct indices of refraction are called birefringent materials.
If the y- and z-directions are equivalent in terms of the crystalline forces, then the x-axis is unique and is called the optic axis of the material. The propagation of light along the optic axis would be independent of its polarization; its electric field, E, is everywhere perpendicular to the optic axis and it is called the ordinary- or o-wave.
The light wave with E-field parallel to the optic axis is called the extraordinary- or e-wave.
Birefringence, B, is defined byB=no−ne,
where no is the ordinary index of refraction; and
ne is the extraordinary index of refraction.
Birefringent materials are used widely in optics to produce polarizing prisms and retarder plates, such as the quarter-wave plate. Putting a birefringent material between crossed polarizers can give rise to interference colors.
A widely used birefringent material is calcite. Its birefringence is extremely large, with indices of refraction for the o- and e-rays of 1.6584 and 1.4864 respectively.
Normally optical waveguides are manufactured to be non-birefringent. Because the typical materials (i.e. fused silica, plastic) used in the fabrication of optical waveguides are homogeneous and isotropic, and the waveguide cross-section is properly shaped, the experimentally observed optical waveguide birefringence is usually small.
In optical waveguides/fibers the birefringence that is experienced by a propagating mode can arise due to three factors:                Lack of π/2 rotational symmetry of the optical waveguide about the axis of propagation, called shape birefringence;        Stress acting transversely on the waveguide creating an optic axis in the direction of the applied stress (even in materials that in unstressed form are homogeneous and isotropic); stress birefringence is used in order to fabricate polarization-maintaining fibers; and        The use of intrinsically birefringent crystalline materials in the fabrication of the waveguide (e.g. Lithium Niobate).Photo-Fabrication of Birefringent Optical Waveguides in Transparent Dielectric Material        
Birefringent optical waveguides can be fabricated within transparent dielectric materials, usually glasses by means of a focused beam of light. Such waveguides exhibit process-controlled levels of optical birefringence.
It is well know that focusing a laser beam in the interior of a dielectric material can change the refractive index of the material in the focal region. M edification of the refractive index of the material occurs when the peak power density of the laser beam at its focus in the material is greater than some peak-power-density threshold value. This threshold is a function of general experimental conditions, the laser wavelength, the optical properties of the material and the pulse duration. The wavelength of the laser is chosen with associated single-photon energy to be less than the absorption band edge of the dielectric material whose index of refraction is to be modified. Thus, the material is transparent at the laser wavelength, as long as power density in the material remains below the characteristic power density threshold.
Efficient multiphoton absorption processes and laser-induced refractive index modification in the focal volume begin to occur when the incident peak power density of the focused beam inside the material exceeds the characteristic threshold for refractive index modification. Typically, the focal volume over which refractive index modification occurs is ellipsoidal in shape, characterized by a waist diameter and a characteristic length. The waist diameter is controlled mainly by the tightness of the laser beam focus and by the absorption-process order, whereas the characteristic length of the ellipsoid depends not only on beam focus and the process order but also on the multiphoton absorption coefficient at the focal point. The dimensions of the focal volume are the order of a few microns.
Relative motion between the focal point and the sample is used to trace out a waveguide, either by translating the sample or by translating the focal point within the sample in a continuous or quasi-continuous manner.
Modification of the index of refraction of the focal volume can be induced by a single pulse from a pulsed laser, by multiple laser pulses acting sequentially on the focal volume or by light from a CW laser. The choice of laser affects the efficiency of the index-modification process. It is clear that modification of the index of refraction of a transparent dielectric material requires that laser energy be absorbed. The laser-beam-induced index of refraction change increases, at least initially, with absorbed energy density, that is the irradiation dose.
There are several lasers available that are suitable for the refractive index modification of transparent dielectric materials. Such lasers include: the F2, KrF, ArF lasers, all UV sources, and femtosecond lasers that operate in the visible and the infrared regions of the spectrum.
The effectiveness of this process hinges on the recognition that when laser energy is absorbed in the focal volume, the temperature in the volume can increase substantially, sometimes reaching several hundred or more Centigrade, for example, with either F2 or femtosecond laser illumination of the sample.
Analytical formulas are available to calculate the temperature rise with pulsed illumination. At these temperatures a dielectric material, such as glass, softens and becomes moldable. Therefore, when the dielectric material is placed under externally applied mechanical stress during laser-induced refractive index modification processing, we anticipate that the mechanical modification will occur in the moldable region of the focal volume. Upon completion of laser irradiation processing, removal of the mechanical stress then leads to a new equilibrium stress distribution in the focal region and its immediate surroundings within the material. The level of externally applied stress applied during illumination in effect becomes “frozen-in”. The material in the focal region retains memory of the magnitude and direction of the applied stress during processing, albeit with a significantly different distribution. We anticipate that the stress in and around the focal volume will reach values similar to those that were present during illumination due to the externally applied mechanical stress.
A dielectric material under tensile or compressive stress exhibits the stress-optic effect, whereby the applied stress changes the index of refraction for light polarized along the direction of applied stress by a different amount than for light polarized at right angles to the applied stress. Thus the applied tensile or compressive stress leads to stress-induced birefringence in the material.
For fused silica we anticipate that the “frozen-in” stress can result in photoinduced birefringence of 1.7×10−4. This value is substantial and can facilitate the fabrication of optical waveguide devices useful in sensor, integrated optics and telecommunications applications. Precise control of birefringence can be helpful in trimming the birefringence of silica-on-silicon integrated optics devices which requires tight control of birefringence levels.
Thus an efficient and practical means is disclosed whereby the application of mechanical stress to the sample affords excellent control of the locally induced residual stress level. This permits us to control both the magnitude and the sign of the birefringence that is “frozen-in”.